1. Field of the Invention
The present invention relates to an apparatus for correction of data representing color information, obtained by analyzing a source image, to obtain corrected color data which can be utilized for example to print a copy of the source image.
2. Description of Prior Art
In recent years, various types of apparatus such as color copiers and color facsimile machines have been developed, in which a source color image (e.g. a color picture) is scanned by a light beam and the reflected light then analyzed to obtain color information representing the source image. In practice, the source image is processed as an array of picture elements, with one set of R, G, B (red, green, blue) primary color data values (referred to in the following as a set of R.sub.a, G.sub.a, B.sub.a values) being obtained for each picture element. The color attributes of each picture element are thereby completely defined, i.e. the hue, chrominance and lightness attributes, where "lightness" signifies a value in a grey-scale range extending from pure white to pure black. These are then corrected in some manner, to obtain a corresponding set of color density data values which are used to determine degrees of density of printing of colors of an output image. In a practical color printer apparatus, color correction processing produces a set of C, M, Y (cyan, magenta, yellow), or C, M, Y, K (cyan, magenta, yellow, black) color density values in response to each set of R.sub.a, G.sub.a, B.sub.a values that is obtained by scanning/analyzing the source image. However in the following it will be assumed for simplicity of description that only red, green and blue primary colors are used, i.e. that red, blue and green density values (R.sub.d, G.sub.d, B.sub.d) are produced by color correction of the R.sub.a, G.sub.a, B.sub.a color analysis values.
The basic operations executed by a color copier apparatus are illustrated in the basic block diagram of FIG. 1. It will be assumed throughout the following specification that the description applies to a color copier apparatus, however the invention is equally applicable to various other types of apparatus such as a color facsimile apparatus, in which color data correction is required. The operation of the apparatus of FIG. 1 consists of sequentially scanning successive portions of an original color image 1, separating the level of reflected light from the image into three primary color components (assumed to be the red, green and blue primary components, designated as R.sub.a, G.sub.a, B.sub.a) by an input section 2, converting the respective levels of these components into respective color density values (also assumed to represent the red, green and blue primary components, designated as R.sub.d, G.sub.d, B.sub.d) by a color correction section 3, and supplying these to an output section 4 which actually executes printing of an output color image. In practice, the levels of the R.sub.d, G.sub.d, B.sub.d values will determine the levels of printer drive signals that are generated in the output section 4, and thereby determine the densities of printing of the respective primary colors. In such a color copier apparatus, primary color separation in the input section 2 is generally executed using dichroic filters, and the respective levels of red, green and blue light which are detected as electrical signals during scanning of the original image are converted into successive sets of R.sub.a, G.sub.a, B.sub.a digital values (i.e. for successive picture elements of the source image 1). All subsequent operations, up to the stage of controlling a printer section for executing a hard-copy print-out, are executed by processing of digital data values. The operation of converting the R.sub.a, G.sub.a, B.sub.a values into corrected R.sub.d, G.sub.d, B.sub.d (or C.sub.d, M.sub.d, Y.sub.d) color density values which are utilized to control color printing is sometimes referred to as masking processing, or masking compensation.
The basic requirement for such a color copier apparatus is that the color attributes of each portion of a print-out image produced from the apparatus should match as closely as possible the color attributes of the corresponding portion of the original image. It is possible to achieve a high degree of accuracy of color separation of the reflected light obtained by scanning the original image; however any colorant that is used in color printing cannot provide a spectrally pure color, but is actually a mixture of colors. For this and other practical reasons, there is a non-linear relationship between the color analysis data obtained from a source image and the color density data that are actually required for accurately controlling reproduction of that image. It is therefore necessary to execute the aforementioned masking correction processing by the color correction section 2 in FIG. 1.
The entire range of possible color conditions that can appear in a source image (i.e. each color condition being a combination of hue, chrominance and lightness attribute values) can be represented as a finite region within a 3-dimensional color space. The problem which has to be solved by the color correction section 3 of FIG. 1 is to produce, in response to each source image picture element R.sub.a, G.sub.a, B.sub.a combination that is inputted thereto, a R.sub.d, G.sub.d, B.sub.d (or C.sub.d, M.sub.d, Y.sub.d, or C.sub.d, M.sub.d, Y.sub.d, K.sub.d) combination which will result in the output section 4 printing a picture element in the output image 5 whose color will approximate as closely as possible that of the source image picture element. Two basic methods have been used in the prior art for achieving this. One method (described for example by Kodera in "Color Reproduction by Digital printer", Image Electronics Conference, May. 14, 1985) is to utilize a set of polynomial equations into which each set of R.sub.a, G.sub.a, B.sub.a values is inserted as a set of variables, and from which a corresponding set of R.sub.a, G.sub.a, B.sub.a (or C.sub.d , M.sub.d, Y.sub.d or C.sub.d, M.sub.d, Y.sub.d, K.sub.d) values is obtained as output. The equations may contain only linear terms, or may contain both linear and non-linear terms. The coefficients of these equations constitute color correction parameters, and can be determined based on computations on the results of experimental measurements, i.e. results obtained by scanning and printing out a large number of color samples, which are distributed within the aforementioned color space. With such a method it is possible to derive a set of coefficients for the polynomial equations such as to ensure that accurate color printing is obtained for each of the color samples (i.e. each having a specific combination of hue, chrominance and lightness). However, in order to achieve a reasonable degree of accuracy for colors which are intermediate between those of the samples, it is necessary to use a very large number of coefficients. This would result in a lengthy computation having to be executed to obtaining color density data for each picture element. In addition, the relationship between each R.sub.a, G.sub.a, B.sub.a set and the corresponding R.sub.d, G.sub.d, B.sub.d (or C.sub.d, M.sub.d, Y.sub.d set) in each range extending between two adjacent color samples (in the 3-dimensional color space) will in general vary in a non-linear manner, and it is not possible to accurately compensate for that non-linearity simply by the aforementioned polynomial equation computatations, since the type of non-linearity will vary in an arbitrary manner in accordance with such factors as the characteristics of the printing inks (which determine, for example, a degree of printing color density change that will occur in response to a change in printer drive signal level), and will also be altered by manufacturing variations of the printer components, etc. Alternatively stated, such a method cannot ensure uniform accuracy of approximation over a wide range of values, within the 3-dimensional color space, between input (R.sub.a, G.sub.a, B.sub.a) and corresponding output (R.sub.d, G.sub.d, B.sub.d, or C.sub.d, M.sub.d, Y.sub.d) color data values. Thus, such a prior art method does not provide a satisfactory degree of accuracy of reproduction over a wide range of source image color values. A color correction method using a function generator and weighting adders has been described by by Yoshida in Japanese Patent No. 57-101840, which has similar disadvantages to those described above.
With a second method that has been proposed in the prior art, a table memory is established, in which is stored a large number of sets of color density (rbgd or C.sub.d, M.sub.d, Y.sub.d) data values, with input (R.sub.a, G.sub.a, B.sub.a) data values being used as address information for read-out from the table memory. However in practice it is necessary to derive the stored values of color density data values by using one of the methods described above, based on computations using a set of fixed coefficients which have been derived by utilizing a limited number of color samples. Thus, such a method will of course have the same disadvantage of a lack of reproduction accuracy that has been described above.